The present invention relates generally to monitoring the production of a web of sheet material and, more particularly, to a method and apparatus for optically scanning a traveling web of sheet material during manufacture to determine properties of the web which are sensitive to infrared radiation (IR) and visible light wherein calibration and preferably compensation for ambient operating conditions are performed as an integral part of the scanning operation. The term "light" will be used generally herein to refer to IR and any other electromagnetic radiation such as visible light which may be used to determine properties of a web of material.
In the art of making sheet material, such as paper, coated paper, plastics and the like, it is important to monitor various characteristics of the sheet material which is typically manufactured as a relatively fast moving web. To be accurate, the monitoring operations must be frequently calibrated and often should be compensated for the ambient environment of the manufacturing process. For example, moisture content of paper is an important IR sensitive characteristic which is measured during manufacture and is highly susceptible to the ambient manufacturing environment, particularly at the wet end of the process.
A popular form for monitoring webs of sheet material traveling in a direction referred to as the machine direction, is to physically move a monitoring gauge across the web in a direction referred to as the cross direction which is substantially perpendicular to the machine direction. Thus, the gauge is moved in an effectively zigzag pattern along the web of material. It is apparent that such monitoring ignores the majority of the web since only the material along the zigzag line is monitored by the traveling gauge. The amount of material which is monitored is further reduced by standard calibration procedures which are performed off web.
Thus, on a periodic basis, such as every thirty (30) minutes, the gauge is moved off the web to an air gap or a calibration sample of the sheet material being monitored. Since the characteristics of the air gap or calibration sample are known, the gauge can be recalibrated during this time. An example of such off sheet calibration where the temperature of the calibration sample is adjusted to that of the manufacturing process is disclosed in U.S. Pat. No. 3,334,230. Even for short calibration periods, it is apparent that a substantial amount of sheet material is not monitored during recalibration. Further, other ambient environment characteristics which influence the manufacturing operation, like surrounding air moisture content, vary over time and are not compensated for by such calibration procedures.
To more completely monitor traveling webs of sheet material during manufacture, optical scanning systems have been developed for the inspection of webs of sheet materials and the determination of light sensitive properties of the webs. One optical scanning system is described in U.S. Pat. No. 3,859,537 wherein a multi-faceted rotating mirror scans laser light across a moving web of material. A photocell 20 is provided at one side of the moving web of material to monitor the intensity of the scanning radiation and provide an alarm when the beam power fails or falls below a minimum acceptable level.
U.S. Pat. No. 3,970,857 describes an optical scanning system wherein a photocell 20 is located on one side of a moving web of material to monitor the intensity of the scanning radiation and a standard swatch 202 of the web material is located on the opposite side of the moving web. The swatch 202 in made to include two or more areas containing defects which are considered objectionable and should be detected by the scanning system. In this way, the optical scanning system is continuously self-checking its own operation to determine when it fails to detect the defects included in the swatch 202.
Large web scanning is described in U.S. Pat. No. 4,260,899 wherein two or more scanners are positioned across a moving web of material and synchronized to fully scan the entire width of the web.
In U.S. Pat. No. 3,843,899, an optical scanning system is described wherein an autocalibrate filter is used to replace the moving web of material at the beginning of the scan. When the scanning beam passes through the autocalibrate filter, a calibration signal is generated for the following scan.
While these optical scanning systems improve over physically moving monitoring gauges across a moving web of material to be monitored, there is a need for further improvements in optical scanning systems to provide economical, reliable scanners which include improved on-line calibration and preferably also compensate for changing ambient operating conditions of the process which is producing the web of material being monitored.